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The Plant Cell, Vol. 21: 507–525, February 2009, www.plantcell.org ã 2009 American Society of Plant Biologists
A Novel Fatty Acyl-CoA Synthetase Is Required for Pollen
Development and Sporopollenin Biosynthesis in Arabidopsis
C W
Clarice de Azevedo Souza,a,1 Sung Soo Kim,a Stefanie Koch,b Lucie Kienow,b Katja Schneider,b,2 Sarah M. McKim,a
George W. Haughn,a Erich Kombrink,b and Carl J. Douglasa,3
a Department
b Max
of Botany, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
Planck Institute for Plant Breeding Research, Department of Plant–Microbe Interactions, 50829 Köln, Germany
Acyl-CoA Synthetase (ACOS) genes are related to 4-coumarate:CoA ligase (4CL) but have distinct functions. The
Arabidopsis thaliana ACOS5 protein is in clade A of Arabidopsis ACOS proteins, the clade most closely related to 4CL
proteins. This clade contains putative nonperoxisomal ACOS enzymes conserved in several angiosperm lineages and in the
moss Physcomitrella patens. Although its function is unknown, ACOS5 is preferentially expressed in the flowers of all
angiosperms examined. Here, we show that an acos5 mutant produced no pollen in mature anthers and no seeds by selffertilization and was severely compromised in pollen wall formation apparently lacking sporopollenin or exine. The
phenotype was first evident at stage 8 of anther development and correlated with maximum ACOS5 mRNA accumulation in
tapetal cells at stages 7 to 8. Green fluorescent protein–ACOS5 fusions showed that ACOS5 is located in the cytoplasm.
Recombinant ACOS5 enzyme was active against oleic acid, allowing kinetic constants for ACOS5 substrates to be
established. Substrate competition assays indicated broad in vitro preference of the enzyme for medium-chain fatty acids.
We propose that ACOS5 encodes an enzyme that participates in a conserved and ancient biochemical pathway required for
sporopollenin monomer biosynthesis that may also include the Arabidopsis CYP703A2 and MS2 enzymes.
INTRODUCTION
Anther development and the development of haploid microspores and mature male gametophytes (pollen grains) is a
complex process central to the angiosperm life cycle. Stages of
anther development and microsporogenesis are precisely timed
and tightly controlled and are characterized by specific events
ranging from initial cell differentiation from the floral meristem to
pollen formation, maturation, and release during anther dehiscence. In tobacco (Nicotiana tabacum) and Arabidopsis thaliana,
anther development has been divided into stages based on anatomical, morphological, cellular, and molecular events (Goldberg
et al., 1993; Sanders et al., 1999; Scott et al., 2004; Ma, 2005).
Molecular genetic studies, particularly in Arabidopsis, have shed
light on events in anther development and microsporogenesis
(Ma, 2005). However, many biochemical and cellular processes
specific to anther development and their regulation are still
uncharacterized.
1 Current
address: Department of Microbiology and Immunology, University of Miami, Leonard Miller School of Medicine, Miami, FL 33136.
2 Current address: Gene Discovery Research Group, Riken Yokohama
Institute, Plant Science Center, Yokohama 230-0045, Japan.
3 Address correspondence to [email protected].
The author responsible for distribution of materials integral to the
findings presented in this article in accordance with the policy described
in the Instructions for Authors (www.plantcell.org) is: Carl J. Douglas
([email protected]).
C
Some figures in this article are displayed in color online but in black
and white in the print edition.
W
Online version contains Web-only data.
www.plantcell.org/cgi/doi/10.1105/tpc.108.062513
One event of fundamental importance during pollen maturation
is the deposition of the pollen wall, necessary for pollen protection, dispersal, and pollen-stigma recognition (Piffanelli et al.,
1998; Scott et al., 2004). The pollen wall is among the most
complex plant extracellular matrices and consists of multiple
layers. A microspore-derived cellulosic primexine is synthesized
by the developing haploid microspores at the tetrad stage
(Blackmore et al., 2007). A thick exine, whose components are
synthesized by the sporophyte, is deposited on the outer surface
of the primexine largely after release of free microspores, and a
pectocellulosic intine is produced by developing microspores
and male gametophytes and deposited internally to the exine
(Blackmore et al., 2007). Since Arabidopsis plants with mutations
in three cellulose synthase CESA genes involved in primary wall
formation are male sterile and exhibit aberrant pollen wall formation, cellulose deposition by the haploid microspores is known to
be important for pollen wall development and pollen fertility
(Persson et al., 2007).
Constituents of the maternally derived exine are produced in
the tapetal cell layer that surrounds the inner surface of the anther
locules (Piffanelli et al., 1998). After secretion into the locules,
exine precursors are deposited and polymerized on the surface
of the primexine, by unknown mechanisms, to form the thick
exine wall that is extremely resistant to degradation (Piffanelli
et al., 1998; Scott et al., 2004; Ma, 2005; Vizcay-Barrena and
Wilson, 2006; Zhang et al., 2006). A primary constituent of the
exine is sporopollenin, an enigmatic polymer of aliphatic and
aromatic constituents coupled via extensive ester and ether
linkages that is extremely resistant to degradation (Scott et al.,
2004; Ma, 2005). The exine not only contributes to the remarkable
resistance of the pollen wall to abiotic and biotic stresses, such as
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The Plant Cell
dehydration, UV irradiation, and pathogen attack, but also facilitates pollen–stigma interactions (Edlund et al., 2004).
In addition to their role in sporopollenin monomer biosynthesis,
tapetal cells are active in the biosynthesis of steryl esters, longchain alkanes, and flavonoids (Hsieh and Huang, 2007). After
synthesis and deposition of the pollen wall, the tapetal cells are
degraded via programmed cell death, resulting in the release of
these constituents into the locule where they form a complex
lipid coat on the outer surface of pollen grains (Piffanelli et al.,
1998; Hsieh and Huang, 2005), the ultimate step in formation of
the pollen grain extracellular matrix.
While recent progress has been made in understanding the
structure and biosynthesis of two other major aliphatic-based
polyester extracellular matrices that share many similarities to
sporopollenin (i.e., cutin and suberin) (reviewed in Franke and
Schreiber, 2007; Pollard et al., 2008), the structure and monomeric composition of the exine and the sporopollenin polymer
remain largely unknown due to the small amounts of material
made during microspore and pollen grain development, the
recalcitrance of exine to chemical degradation, and the limited
number of techniques available for exine chemical analysis
(Dominguez et al., 1999; Bubert et al., 2002; Ahlers et al., 2003;
Blokker et al., 2005). Available data from several plants indicate
that the major monomeric components of the sporopollenin
polymer are polyhydroxylated, unbranched long-chain, and/or
very-long-chain fatty acids, as well as oxygenated aromatic
compounds, likely derived from phenylpropanoid acids, such as
p-coumaric, caffeic, and/or ferulic acids (Guilford et al., 1988;
Ahlers et al., 1999; Dominguez et al., 1999; Ahlers et al., 2000;
Blackmore et al., 2007). While the nature of the sporopollenin
structure and the monomer building blocks has not been clearly
defined, data from chemical degradation of isolated sporopollenin suggest that there are a limited number of monomeric
constituents (Bubert et al., 2002). It has also been recognized
that there are chemical similarities between the walls of spores in
the descendants of early land plants, such as bryophytes and
lycopods, and those of pollen grains (Morant et al., 2007). Thus, a
key innovation in the colonization of the terrestrial environment
by the first land plants 600 to 450 million years ago may have
been the acquisition of the ability to generate the sporopollenin
polymer to protect haploid spores (Bowman et al., 2007; Morant
et al., 2007), and sporopollenin may have been one of the first
plant polyester-based extracellular matrices to have evolved
(Morant et al., 2007).
Numerous male sterile or partially sterile mutants that have
been isolated and characterized in Arabidopsis define some of
the key processes in anther and male gametophyte development
(Taylor et al., 1998; Sanders et al., 1999; Ma, 2005). Male sterile
mutants displaying apparently normal microspore and tapetum
development, but with defects in pollen wall formation and pollen
maturation, have started to shed light on the cell biology and
biochemistry of pollen wall, exine, and sporopollenin development and biosynthesis. For example, the male sterile2 (ms2)
mutant, defective in sporopollenin deposition, develops microspores that collapse shortly after release from tetrads, showing
no signs of pollen wall formation. The MS2 protein accumulates
specifically in the tapetum and was suggested to be a long-chain
fatty acid reductase that is involved in the biosynthesis of a long-
chain aliphatic component of the sporopollenin polymer (Aarts
et al., 1997). In the defective in exine formation1 (dex1) mutant,
abnormal microspores develop after release from tetrads and
eventually collapse, as in the ms2 mutant, because the sporopollenin that is produced does not anchor to the developing
pollen grains (Paxson-Sowders et al., 2001). DEX1 is a novel
protein of unknown function thought to be localized to the
plasmalemma. Other genes associated with exine production
and male fertility include YORE-YORE/WAX2/FACELESSPOLLEN1, encoding an enzyme apparently involved in wax biosynthesis (Ariizumi et al., 2003; Chen et al., 2003; Kurata et al., 2003)
and the rice (Oryza sativa) Wax-deficient anther1 gene, a homolog of the Arabidopsis ECERIFERUM1 gene that encodes a
putative membrane protein apparently required for very-longchain fatty acid metabolism (Jung et al., 2006).
Recently, the Arabidopsis cytochrome P450 enzyme CYP703A2
has been shown to be required for the biosynthesis of sporopollenin precursors. CYP703A2 knockout mutants (dex2) are partially male sterile, with poorly developed exine, and apparently
lack sporopollenin (Morant et al., 2007). CYP703A2 appears to
function in vitro as a fatty acid in-chain hydroxylase that preferentially generates 7-hydroxy lauric acid, which is proposed to be
a sporopollenin precursor (Morant et al., 2007). The conservation
of the CYP703 clade in bryophytes and other land plant lineages
suggests that the enzyme participates in an ancient, conserved
biochemical pathway for sporopollenin biosynthesis (Morant
et al., 2007).
Novel classes of Arabidopsis genes encoding enzymes related
to, yet functionally distinct from, phenylpropanoid genes have
been defined (phenylpropanoid-like genes; Costa et al., 2003;
Raes et al., 2003; Ehlting et al., 2005). Many such genes are
conserved in the fully sequenced genomes of poplar (Populus
trichocarpa) and rice (Oryza sativa) (Tuskan et al., 2006;
Hamberger et al., 2007; de Azevedo Souza et al., 2008). An
example of such a supergene family is the group of genes
encoding adenylate-forming enzymes related to the phenylpropanoid enzyme 4-coumarate:CoA ligase (4CL) (Cukovic et al.,
2001; Costa et al., 2003, 2005; Schneider et al., 2003, 2005;
Shockey et al., 2003; Ehlting et al., 2005; de Azevedo Souza
et al., 2008). Recent biochemical characterizations of enzymes
encoded by this family of 4CL-like genes show that they generally
do not convert substrates accepted by true 4CL enzymes to the
corresponding CoA esters (Costa et al., 2005; Kienow et al.,
2008). Instead, many accept fatty acyl substrates, such as
12-oxo-phytodienoic acid (OPDA) and OPDA derivatives (jasmonic acid precursors; Koo et al., 2006; Kienow et al., 2008), and
medium- to long-chain fatty acids (Kienow et al., 2008). Based on
such activities, we have designated those 4CL-like genes with
unknown in vivo substrates as Acyl-CoA Synthetase (ACOS;
formerly referred to as the ACS class of genes; de Azevedo
Souza et al., 2008). This class of genes is distinct from those
encoding long-chain acyl-CoA synthetases (LACS; Shockey
et al., 2002), such as the Arabidopsis LACS2 gene, which is
required for cutin biosynthesis (Schnurr et al., 2004), or the
LACS6 and LACS7 genes, involved in storage lipid degradation
(Fulda et al., 2004). A recent genome-wide phylogenetic analysis
of ACOS genes shows that variable numbers of ACOS homologs
are present in the poplar, rice, and Physcomitrella patens
ACOS5 in Pollen Wall Biosynthesis
genomes but that they are absent from Chlamydomonas reinhardtii (de Azevedo Souza et al., 2008).
Most ACOS genes identified in Arabidopsis and other species
are predicted to localize to peroxisomes, based on the presence
of C-terminal peroxisomal targeting sequences (Costa et al.,
2005; Schneider et al., 2005; de Azevedo Souza et al., 2008), and this
is consistent with their having roles in fatty acid and jasmonate
metabolism, which occur in this cellular compartment (Schneider
et al., 2005; Koo et al., 2006; Kienow et al., 2008). One ACOS
clade identified in this analysis is unique in that it contains singlecopy genes from Arabidopsis, poplar, and rice, which all encode
enzymes without predicted peroxisomal targeting sequences (de
Azevedo Souza et al., 2008). The Arabidopsis gene in this clade,
ACOS5, is preferentially expressed in flowers (Douglas and
Ehlting, 2005; de Azevedo Souza et al., 2008), whereas expression of the poplar ortholog is mainly in male flowers (de Azevedo
Souza et al., 2008). Thus, a function for ACOS5 in anther development has been proposed (de Azevedo Souza et al., 2008).
In this study, we characterized a loss-of-function allele of the
Arabidopsis ACOS5 gene, acos5-1, and showed that acos5-1
homozygotes are completely male sterile, with no pollen observed
in mature anthers and no seeds produced by self-fertilization.
Further analyses demonstrated that ACOS5 is a medium- to longchain fatty acyl-CoA synthetase that is required in tapetal cells for
sporopollenin monomer biosynthesis. Light and transmission
electron microscopy studies indicated that the defect in pollen
development in the acos5 mutant coincides with exine deposition
at the unicellular stage, which is completely lacking in the mutant.
Our data indicate that ACOS5 encodes an acyl-CoA synthetase
protein that participates in an ancient biochemical pathway
required for sporopollenin monomer biosynthesis, providing
novel insight into the reactions underlying the biosynthesis of
this important polymer.
RESULTS
Arabidopsis ACOS5 Is a Member of an Ancient and
Conserved Clade of Nonperoxisomal 4CL-Like Genes
Previous results showed that the Arabidopsis 4CL-like gene
ACOS5 is a single-copy gene with homologs in the genomes of
representative fully sequenced land plants (de Azevedo Souza
et al., 2008). To further clarify the relationship between ACOS5, its
homologs, and true 4CL genes in plants, we used the corresponding protein sequences from Arabidopsis, poplar, rice, the
moss Physcomitrella, and tobacco (Nicotiana sylvestris) (as listed
in Table 1; see Supplemental Table 1 online) to construct an
unrooted maximum likelihood phylogenetic tree (Figure 1; see
Supplemental Data Set 1 online). In this phylogenetic reconstruction, we included homologs from each of the plants examined and found that clade A ACOS genes form a distinct
phylogenetic sister group to 4CLs. In contrast with bona fide
4CL genes, which have undergone complex patterns of gene
family expansion in different lineages (Figure 1), clade A ACOS
proteins are encoded by single-copy genes in each of the four
species analyzed for which the complete genome sequences are
available. Also, according to this analysis, the Physcomitrella
clade A ACOS gene (ACOS6) and the four Physcomitrella sequences that encode 4CL proteins (Silber et al., 2008) are basal to
the angiosperm clade A ACOS genes and to angiosperm 4CL
genes, respectively (Figure 1). This suggests that both 4CL and
clade A ACOS genes arose early in land plant evolution and were
present in the most recent common ancestor of angiosperms and
Physcomitrella. Consistent with this, no clade A ACOS genes
were found in the C. reinhardtii genome (Table 1).
Unlike most other 4CL-like ACOS proteins, Arabidopsis ACOS5
and its homologs in other species do not contain recognizable
peroxisomal targeting sequences of type 1 at their C termini (de
Azevedo Souza et al., 2008). To empirically test the subcellular
localization of ACOS5, we generated a yellow fluorescent protein
(YFP)-tagged variant that was placed under the control of the
cauliflower mosaic virus 35S promoter (p35S:YFP-ACOS5) and
transiently expressed this construct together with a reporter
construct of a peroxisomal marker protein (p35S:RFP-SRL reporter) (Schneider et al., 2005). Red fluorescent protein (RFP) and
YFP localization in single Arabidopsis epidermal cells was assayed
by confocal laser scanning microscopy using appropriate filter
sets for simultaneous and selective recording of RFP and YFP
(Figure 2). While RFP-SRL–expressing cells displayed a punctate
pattern typical of peroxisomal localization, YFP-ACOS5 localization did not coincide with this pattern; instead, YFP fluorescence
was primarily restricted to the cytoplasm (Figure 2). This suggests
that the ACOS5 enzyme is localized to the cytoplasm.
Table 1. Putative ACOS5 Orthologs and Expression in Other Species
Species
Gene name
Accession or Gene Model
Expression
Arabidopsis
P. trichocarpa
O. sativa
N. sylvestris
P. patens
C. reinhardtii
ACOS5
ACOS13
ACOS12
CL-lk
ACOS6
No hit
At1g62940
eugene3.00010460
Os04g24530
AY163489
fgenesh1_pg.scaffold_96000119
Tapetuma
Male flowerb
Immature paniclec
Tapetumd
NAe
aThis
study.
de Azevedo Souza et al. (2008).
cFrom http://mpss.udel.edu/rice/.
dFrom Varbanova et al. (2003).
eNA, no information available.
bFrom
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Figure 1. Phylogenetic Analysis of Selected 4CL and 4CL-Like ACOS Protein Sequences.
Sequences from Arabidopsis, poplar, rice, Physcomitrella, and Nicotiana were aligned and used to construct an unrooted maximum likelihood tree
using PhyML 2.4.4. Bootstrap values (1000 replicates) above 70% are shown on branches. The protein encoded by an N. sylvestris gene expressed in
tapetal cells during anther development is indicated by a flash. Class I and II 4CL subclades as defined by Ehlting et al. (1999) and a Physcomitrella 4CL
subclade are indicated by boxed text. Genes used in this analysis are given in Supplemental Table 1 online, and the sequences used to generate this
phylogeny are provided in Supplemental Data Set 1 online. Bar = 0.1 amino acid substitutions.
Identification of an ACOS5 Loss-of-Function Allele
Previous experiments showed that Arabidopsis ACOS5 has a
flower-preferred expression pattern, that the poplar homolog
ACOS13 is expressed in male but not in female flowers (de
Azevedo Souza et al., 2008), and that the N. sylvestris homolog
CL-lk is expressed specifically in the tapetum of developing
anthers (Varbanova et al., 2003). To explore the organ- and cell
type–specific expression pattern of ACOS5, promoter sequences of variable length (1030, 1416, and 1620 bp) were fused
to the b-glucuronidase (GUS) reporter gene and integrated into
the Arabidopsis genome. Several independent transgenic lines
homozygous for a single copy of the ProACOS5:GUS gene fusion
were analyzed for GUS activity. All lines yielded identical results,
and representative data are shown in Figure 3. No GUS expression was observed in vegetative organs of seedlings and plants
(i.e., cotyledons, leaves, roots, and stems; Figures 3A and 3B).
However, GUS expression was first evident in young flower buds
at developmental stages 7 to 9 according to Sanders et al. (1999)
where it was restricted specifically to the developing anthers
(Figures 3D to 3F). Beyond stage 9, GUS expression rapidly
declined, with no GUS staining being detectable at anther maturity (Figure 3C).
We obtained an ACOS5 transposon insertion line (SM_3.37225)
from the Nottingham Arabidopsis Stock Centre (NASC). Within
the segregating population, plants homozygous for the ACOS5
insertion allele (acos5-1) were identified by PCR genotyping, and
the location of the transposon insertion in the second exon of
ACOS5 was verified (Figure 4A). We assayed ACOS5 expression
by RT-PCR in both wild-type and mutant flowers. Using primers
flanking the insertion site, no ACOS5 mRNA could be detected in
the mutant, indicating that functional ACOS5 mRNA does not
accumulate in the mutant and that acos5-1 is likely to be a null
allele of ACOS5 (Figure 4A).
Initial phenotypic analysis of the homozygous acos5-1 plants
suggested that the plants were self-sterile, since siliques failed to
mature and produce seeds. Careful examination of the mutant
flowers revealed anthers that were apparently devoid of pollen
grains (Figure 5A). At a time when flowers of wild-type plants
grown in parallel had self-pollinated and produced siliques with
developing seeds, development of acos5 mutant flowers culminated in undeveloped siliques and complete absence of seed
production (Figure 5A). There were no other obvious morphological differences between the acos5 mutant and wild-type
plants grown to maturity (Figure 5A), except that acos5 mutant
plants flowered for a longer time (Figure 5A), consistent with
observations of other male sterile mutants (Aarts et al., 1997).
Secondary wall formation and lignin deposition in anther
endothecium cells is required for anther dehiscence and pollen release (Dawson et al., 1999; Yang et al., 2007). Given the close
relationship between ACOS5 and 4CL genes, we considered
ACOS5 in Pollen Wall Biosynthesis
511
Figure 2. Subcellular Localization of ACOS5 in Arabidopsis Leaf Epidermal Cells.
(A) Structural features of the reporter constructs p35S:RFP-SRL, with the synthetic C terminus –SRL serving as peroxisomal marker, and p35S:YFPACOS5 both under the control of the cauliflower mosaic virus 35S promoter (p35S).
(B) and (C) Both expression plasmids were cotransformed into Arabidopsis leaves by particle bombardment. Single epidermal cells were inspected by
confocal laser scanning microscopy using appropriate filter sets for simultaneous and selective recording of RFP (B) and YFP (C) fluorescence. Optical
sections were acquired for the reconstruction of three-dimensional images of the cellular labeling patterns and indicate that the YFP signal is primarily
restricted to the cytoplasm and does not coincide with the punctate RFP fluorescence pattern of the peroxisomal marker (p). Bar = 25 mm.
the possibility that the enzyme encoded by ACOS5 is required
for lignin biosynthesis in the endothecium and that lack of lignification and anther dehiscence resulted in the loss of pollen release.
To test this, we treated anthers in developing wild-type and
acos5-1 mutant flowers with phloroglucinol, which stains lignin and
other wall-bound phenolics. However, anthers from both wild-type
and acos5-1 mutant plants stained equally well, suggesting that
there is no defect in lignin deposition in the mutant and that
ACOS5 activity is not required for lignin biosynthesis in the anther
(Figure 5B).
To investigate the basis for the failure of self-pollination in
acos5 mutant plants, we used scanning electron microscopy to
view anther development at higher resolution in wild-type and
mutant flowers. acos5 mutant anthers underwent apparently
normal development and dehiscence, but no pollen grains could
be observed either in dehiscent anthers or attached to stigmata;
by contrast, abundant pollen was observed in dehiscing wildtype anthers examined in parallel (see Supplemental Figure
1 online). The absence of pollen in acos5 mutant flowers, as
observed by light or scanning electron microscopy, indicates that
ACOS5 function is absolutely required for pollen maturation.
When acos5 mutant plants were used as pollen recipients in
crosses with pollen from wild-type plants, siliques developed
normally and produced F1 seeds that germinated normally with
no apparent loss in fecundity relative to self-pollinated wild-type
plants.
Since only one insertional mutant line was available for ACOS5,
we allowed the F1 heterozygote plants derived from the above
cross to self-pollinate to test the resulting F2 population for
cosegregation of the male sterile phenotype with acos5-1. In this
population, the male sterile mutant phenotype was inherited in a
Mendelian fashion, with one-quarter of the 184 F2 progeny
displaying male sterility (x2 = 0.437; P > 0.4; n = 184), illustrating
that the mutant phenotype is caused by a mutation at a single
locus. Next, we determined the genotypes of 183 F2 plants. This
analysis showed that the male sterile phenotype cosegregated
with acos5-1 (51/51 acos5-1 homozygotes were male sterile,
whereas all 132 individuals that were either ACOS5/acos5-1 heterozygotes or ACOS5 homozygotes were fertile). This strongly
suggests that the male sterile phenotype and the complete block
of pollen formation were caused by loss of function of the ACOS5
gene.
To test for the ability of the ACOS5 gene to complement the
male sterile phenotype in the acos5-1 background, we introduced an ;4.4-kb DNA region from an Arabidopsis wild-type
(Columbia-0 [Col-0]) plant, containing 1.9 kb of the promoter
sequence and the complete transcribed region of ACOS5 (Figure
4B), into ACOS5/acos5-1 heterozygote plants by Agrobacterium
tumefaciens–mediated transformation. Four T1 lines harboring
the ACOS5 transgene were subjected to PCR-aided genotyping,
and one was established as being ACOS5/ACOS5, one as being
ACOS5/acos5-1, and two as being acos5-1/acos5-1. All plants
were fully fertile, suggesting that the introduced ACOS5 transgene had complemented the acos5-1 mutation in the two homozygous lines. We further determined the genotypes and
phenotypes of 18 T2 progeny from each T1 line that had inherited
the ACOS5 transgene (i.e., that was either homozygous or
hemizygous for the ACOS5 transgene, based on hygromycin
resistance specified by the T-DNA insertion). For each T1 line,
including the two that were homozygous for the acos5-1 allele
and gave rise only to acos5-1/acos5-1 T2 progeny, all 18 T2
plants were fully fertile, confirming the ability of ACOS5 to
complement the male sterile mutant phenotype.
Anther and Microspore Development in the acos5-1 Mutant
To identify the point at which pollen development was impaired in
the acos5 mutant, we analyzed anther cross sections taken from
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seen in the locules of wild-type anthers, stage 9 mutant locules
were filled with misshapen structures.
During stage 10, wild-type pollen grains continued to enlarge
and develop, and the tapetum layer showed initial signs of
degeneration. By contrast, in the mutant anthers at an age
equivalent to stage 10, we observed degradation of both microspores and the tapetum (Figure 6). Thus, in the acos5 mutant, no
developing pollen grains were observed at this stage, and the
tapetum degenerated earlier than in wild-type plants. At stage 11,
the tapetal cell layer was greatly degraded in wild-type anthers,
and clear darkly staining pollen grains were seen. By contrast,
although fully mature anthers appeared otherwise normal in the
mutant, the locules were empty, devoid of pollen grains (Figure 6).
These data suggest that the acos5 male sterile phenotype is
manifested relatively late in anther and microspore development,
at stages 8 and 9 (free microspore stages) when exine formation
and sporopollenin deposition occur.
To further pinpoint the stage of anther development defective in
the acos5 mutant, we used transmission electron microscopy to
gain higher-resolution images of developing anthers in the acos5
mutant and compared these to the corresponding images from
wild-type plants. Figures 7A to 7D show that, at stage 7, characterized by the presence of tetrads that form after meiosis, wild-type
and acos5 mutant microspores were similar in morphology, with
Figure 3. Developmental and Tissue-Specific Expression of the ACOS5
Gene.
Arabidopsis lines homozygous for a single insertion of the reporter gene,
comprising a 1030-bp promoter of the ACOS5 gene and the GUS
reporter, were generated and selected. Reporter activity was revealed by
X-gluc staining. Stages are according to Sanders et al. (1999). Bar = 1000
mm in (A), 500 mm in (B), and 150 mm in (C) to (F).
(A) Leaf of three-week-old plant grown on soil.
(B) One-week-old seedling grown on Murashige and Skoog medium.
(C) Fully developed flower at anthesis (stage 12 to 13).
(D) to (F) Young flower buds (stage 7 to 9).
wild-type (Col-0) and acos5 flowers at different stages of anther
development, as defined by Sanders et al. (1999). Representative sections are shown in Figure 6. Early stages of anther
development in acos5 appeared normal. By stage 5, in both
mutant and wild-type anthers, four defined locules were established and visible pollen mother cells had appeared. Subsequently, the pollen mother cells underwent meiosis and tetrads
were formed, connected by a callose wall (stage 7) in both wildtype and acos5 plants. In both wild-type and acos5 mutant
anthers, individual microspores could be seen at stage 8, indicating that the callose wall had degenerated, releasing the
microspores from the tetrads in a normal manner.
At stage 9, the wild-type pollen grains became vacuolated,
and an exine wall started to become visible, as evidenced by
toluidine blue staining of the pollen grain walls. However, normal
development seemed to be arrested in the acos5 mutant at this
stage. Although the vacuoles were evident in some mutant
microspores, microspores appeared malformed and distorted
in shape, and exine walls were not as clearly evident as in the wild
type (Figure 6). In place of the clearly developing microspores
Figure 4. Molecular Characterization of the acos5-1 Insertion Allele,
Effects of the Insertion on Gene Expression, and the Construct Used for
Complementation.
(A) Location of the transposon insertion in the second exon of ACOS5 in
the acos5-1 allele is shown above. Black boxes denote exons, whereas
thin horizontal lines denote introns. Below, RT-PCR analysis of ACOS5
expression in wild-type and acos5 flowers is shown. Expression was
evaluated using intron-spanning primers on either side of the T-DNA
insertion as described in Methods. Two biological replicates of the
analysis are shown and gave identical results.
(B) Schematic representation of the construct used for the asoc5 complementation test. Shaded boxes show the ACOS5 transgene, containing
1.9 kb of the promoter sequence, the 39 untranslated region/terminator
regions (light gray), and the transcribed region between the start and stop
codons (thick black arrow). Primers used to amplify genomic DNA are
indicated with arrows above and below the diagram and are shown in
Supplemental Table 4 online.
ACOS5 in Pollen Wall Biosynthesis
513
Figure 5. Phenotypic Characterization of acos5 Plants.
(A) Mature wild-type (Col-0) and acos5 flowers are shown. Mutant anthers are devoid of pollen, and no pollen grains were attached to the stigma.
Siliques devoid of seeds derived from acos5 homozygotes relative to the wild type (Col-0) are shown below. To the right, the morphology of mature wildtype (Col-0) and acos5 mutant plants is shown
(B) Lignin and wall-bound phenolic accumulation in wild-type and acos5 anthers. Mature wild-type (Col-0) and acos5 flowers were stained with
phloroglucinol and showed similar staining patterns in anthers.
characteristic callose walls. At this stage, tapetal cells in the
mutant appeared normal. At stage 8, when free microspores had
been released from tetrads following callose wall hydrolysis,
massive deposition of a thick sporopollenin-containing exine on
the nexine layer was evident in the wild-type anthers (Figures 7E
and 7F). The exine had started to develop into a thick, reticulated
wall characteristic of pollen grains, residual primexine was visible
in spaces of exine baculae, which are characteristic of this stage
(Goldberg et al., 1993; Sanders et al., 1999; Scott et al., 2004; Ma,
2005), and microspores were nonvacuolated. In comparison with
the wild type, free acos5 mutant microspores at stage 8 had a
similar nonvacuolated morphology but contained thinner walls that
were devoid of the pronounced reticulate exine wall seen in the
wild type (Figures 7G and 7H). In place of a well-defined exine,
these microspores contained an amorphous substance adhering
to the nexine that could be residual primexine or unpolymerized
sporopollenin precursors. Again, tapetal cells at this stage appeared normal.
At stage 9, wild-type pollen grains contained thick, reticulated
exine walls and an intine layer. In stage 9 of the acos5 mutant,
many pollen grains were in various stages of lysis and degradation (Figure 7K) and had thin cell walls that were devoid of a
characteristic exine layer (Figure 7L) but often retained amorphous wall material outside the intine and apparent nexine. In
other cases, relatively intact pollen grains were observed in acos5
anthers at stage 9 (Figure 7M), but in many cases, pollen walls
were completely devoid of wall material outside the intine (Figure
7N). Again, tapetal cells in the acos5 mutant anthers appeared
normal at this stage (Figures 7K and 7M), suggesting that the
defect in acos5 pollen development is primarily due to the lack of
sporopollenin deposition and exine formation, rather than a
general defect in tapetal cell development. This analysis pinpoints the defect in microspore development in the acos5 mutant
to the deposition of the sporopollenin-rich exine wall at stages 8
and 9 of anther development, while other aspects of anther
development, including tapetal cell development, appear normal.
Localization of ACOS5 Expression in Developing Anthers
To gain insight into the spatio-temporal pattern of ACOS5 expression in anthers, we performed in situ hybridization using an
ACOS5-derived probe and developing wild-type flowers. Figure
8A shows that ACOS5 was strongly, specifically, and transiently
expressed in the tapetal cell layer of developing anthers. At the
beginning of stage 5, locules with microspore mother cells and a
distinct tapetum were evident, but no hybridization signal was
observed. Toward the end of stage 5, clear but relatively weak
hybridization of the probe to the developing tapetum was
detected. By stage 7, which is characterized by the presence of
tetrads within the locules, maximal hybridization was seen in the
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Figure 6. Phenotypic Characterization of Anther and Microspore Development in Wild-Type (Col-0) and aco5 Flowers.
Anther cross sections (1 mm) were taken from developing flowers of the indicated genotypes. Numbers indicate anther developmental stages according
to Sanders et al. (1999). Anther development in acos5 plants appeared normal until stage 8 (i.e., release of free microspores). At stage 9, initial
degradation of microspores was observed, but the tapetal cell layer appeared normal relative to the wild type. By stage 10, microspore degradation was
complete in acos5 plants, whereas maturing pollen grains were observed in wild-type plants. Mature mutant anthers appeared normal but were devoid
of pollen grains. Bars = 40 mm.
[See online article for color version of this figure.]
tapetum. By stage 8, during which microspores were released
from tetrads, hybridization to tapetal cells weakened and disappeared by stage 11. Similarly, GUS reporter activity was
restricted to the tapetum in transgenic lines harboring the 1030bp ProACOS5:GUS fusion (Figure 8B). Collectively, these results
demonstrate that ACOS5 expression is highly specific to tapetal
cells and is highest in the stages immediately preceding the
appearance of the visible phenotype at stage 8. The results are
consistent with an ACOS5 function in tapetal cells, which are
known to be the sites for the biosynthesis of sporopollenin
precursors required for exine formation in developing microspores (Goldberg et al., 1993; Sanders et al., 1999; Scott et al.,
2004; Ma, 2005).
Biochemical Function of ACOS5
We previously described the heterologous expression of ACOS5
in Escherichia coli and its application in a substrate/activity
screen using an ATP depletion assay (Kienow et al., 2008).
Among >80 potential substrates tested, in vitro activity was
observed only against several long-chain fatty acids (i.e., octadecanoic, oleic, linoleic, and linolenic acids), suggesting that
ACOS5 possibly functions as a fatty acyl-CoA synthetase. This
activity was confirmed in vivo by the capacity of ACOS5 to
complement the deficient acyl-CoA synthetase in the E. coli fadD
mutant by rescuing growth on oleic acid (C18:1) (Kienow et al.,
2008). Further analysis showed that ACOS5 could also rescue
growth on saturated medium-chain fatty acids of chain length C8
to C14, whereas fadD growth on shorter-chain fatty acids (C6
and C4) was not supported. To extend these results, we used a
more sensitive radioactive assay to directly and unequivocally
demonstrate activity of recombinant ACOS5 against fatty acid
substrates in vitro. When using 14C-labeled oleic acid as a
substrate (20 mM), the affinity-purified recombinant ACOS5
protein displayed specific activities of CoA-ester formation
ranging from 36 to 50 pkat/mg protein. Although this activity is
ACOS5 in Pollen Wall Biosynthesis
Figure 7. Transmission Electron Micrographs of Wild-Type (Col-0) and acos5 Mutant Anthers, Pollen Grains, and Pollen Walls.
515
516
The Plant Cell
Figure 8. Tapetum-Specific Expression of ACOS5.
(A) ACOS5 mRNA was localized by in situ hybridization to sections taken from developing anther locules of wild-type (Col-0) flowers using a genespecific antisense probe and a control sense probe. Stages of anther development are according to Sanders et al. (1999). Dark precipitates indicate
hybridization of the probe. From left to right: early stage 5 locules with microspore mother cells (MMCs) and a distinct tapetum (T); late stage 5 locules
show a hybridization signal in the tapetum; by stage 7, indicated by tetrads (Tds) within the locule, maximal hybridization is detected in the tapetum; the
signal in the tapetum weakens by stage 8 and disappears by stage 11; and hybridization with the sense control probe produces no signal. MSp,
microspores; PG, pollen grain. Bars = 70 mm.
(B) Tapetum-specific expression of an ACOS5 promoter-GUS fusion. Arabidopsis lines homozygous for a single insertion of the reporter gene,
comprising a 1030-bp promoter of the ACOS5 gene and the GUS reporter, were generated and. Reporter activity was revealed by X-gluc staining of
longitudinal sections of developing anthers. Bars = 80 mm.
low, it is comparable to that of other plant fatty acyl-CoA
synthetases expressed in E. coli (Fulda et al., 1997; Schnurr
et al., 2002, 2004; Shockey et al., 2003). Furthermore, activity
was dependent on oleic acid, CoASH, ATP, and native enzyme.
The identity of the oleic acid CoA-ester product was verified by
mass spectrometry (see Supplemental Table 2 online). The
enzyme showed Michaelis-Menten kinetics toward all of substrates, and the derived kinetic parameters are listed in Table 2.
The Km values in the low micromolar range for ACOS5 substrates
have also been reported for other acyl-CoA synthetases (Pongdontri
and Hills, 2001; Schnurr et al., 2004; Oba et al., 2005).
To test for the in vitro preference of ACOS5 for different fatty
acid substrates, we applied a competition assay, which determined the conversion of oleic acid (20 mM) to the corresponding
CoA ester in the presence of other potential substrates that acted
as competitors. As predicted, addition of unlabeled oleic acid at
concentrations of either 50 or 100 mM to the standard reaction
mixes decreased the apparent rate of formation of the labeled
reaction product to 27 and 15%, respectively (Figure 9A). Among
the saturated fatty acids, ACOS5 showed clear preference for
those of medium-chain length (C10 to C14), which competed
nearly as well as oleic acid, whereas fatty acids of longer or
shorter chain length competed less efficiently (Figures 9A and
9B). When hydroxy fatty acids differing in chain length and
substitution pattern were compared, large differences in competition activity were observed. Among the hydroxy-octanoic
acids, 8-hydroxy octanoic acid (8OH-C8) competed better than
oleic acid, whereas 2OH-C8 and 3OH-C8 were as inactive as
Figure 7. (continued).
(A), (B), (E), (F), (I), and (J) Microspore and pollen wall development in Col-0 wild-type plants.
(C), (D), (G), (H), and (K) to (N) Microspore and pollen wall development in acos5 mutants.
(A) to (D) Stage 7 anthers.
(E) to (H) Stage 8 anthers.
(I) to (N) Stage 9 anthers.
White arrows in (F) indicate residual primexine in developing baculae of the exine. CW, callose wall; Ex, exine; In, intine; Lo, locule; Msp, microspore; Ne,
nexine; Nu, nucleus; PM, plasma membrane; T, tapetal cell; Td, tetrad. Bars = 2 mm in (A), (C), (E), (G), (I), (K), and (M) and 500 nm in (B), (D), (F), (H), (J),
(L), and (N).
ACOS5 in Pollen Wall Biosynthesis
Function of ACOS5 in Tapetal Cells
Table 2. Kinetic Properties of ACOS5
Substrate
Km (mM)
Oleic acid
3.26 6 1.30
Coenzyme A 1.46 6 0.29
ATP
35.45 6 6.18
Vmax
kcat
(pkat mg 1) (s 1 10 3)
517
kcat/Km
(s 1 M 1)
49.2 6 9.3 2.91 6 0.55 892 6 169
43.5 6 6.7 2.57 6 0.39 1762 6 270
51.0 6 13.5 3.02 6 0.80
85 6 23
Enzyme activity was determined by a radiochemical assay with affinitypurified protein and various concentrations of the substrates listed. All
values are the mean 6 SD of at least three independent determinations.
unsubstituted octanoic acid or 5-dodecanolide, which served as
a negative control (Figure 9C). Thus, the position of the hydroxyl
group is critical for activity. Among the long-chain hydroxy fatty
acids tested, 12OH-C18 and 16OH-C16 were the most efficient
competitors, the latter exceeding the apparent effectiveness of
oleic acid, whereas medium-chain hydroxy fatty acids (10OHC10 and 12OH-C12) were again less active (Figure 9D). Intriguingly, the oxylipin OPC-8 impaired oleic acid conversion quite
efficiently, whereas none of the hydroxycinnamic acids were
effective above control values, with the possible exception of
caffeic acid (Figure 9E). We confirmed the ability of ACOS5 to
generate CoA esters of the hydroxylated fatty acids 8OH-C8 and
16OH-C16 by mass spectrometric analysis of CoA-esters formed
in vitro by incubation of ACOS5 with the corresponding acids (see
Supplemental Figure 2 online). From these results we conclude
that ACOS5 is a medium- to long-chain fatty acyl-CoA synthetase
with a preference for substituted fatty acids carrying a hydroxy
group at position C8 to C16, whereas activity as a 4CL can be
dismissed.
DISCUSSION
In this study, we used reverse genetics to define the functions of
a novel gene essential for male fertility and postmeiotic pollen
development in Arabidopsis and identified putative orthologs in
other species, including Physcomitrella. ACOS5 encodes an
acyl-CoA synthetase with in vitro activity toward medium- to
long-chain fatty acids, their hydroxylated derivatives, and other
fatty acyl-containing substrates, and it appears to be required for
sporopollenin biosynthesis. In the acos5 loss-of-function mutant,
pollen development is arrested after release from tetrads and
free microspores are devoid of exine (Figures 6 and 7). A unique
feature of the mutant is the complete absence of pollen grains at
anther maturity and consequent complete male sterility. While a
number of other Arabidopsis male sterile mutants with strongly
impaired exine and pollen wall development have been described (Aarts et al., 1997; Morant et al., 2007; Persson et al.,
2007), such mutants typically retain variable amounts of (abnormal) pollen grains at anther maturity and corresponding low
levels of fertility and seed development. By contrast, the acos5
homozygote line never produced even partially fertile siliques,
necessitating maintenance of the mutant lines as heterozygotes.
The exceptionally strong male sterile phenotype and lack of
exine formation suggests that ACOS5 plays a crucial role in exine
formation and sporopollenin biosynthesis.
The tapetum is an endothelial cell layer highly specialized for the
biosynthesis and secretion of maternally derived pollen wall
components, such as sporopollenin. The aliphatic and phenolic
monomers of sporopolleninin are coupled by ether and ester
linkages, which contribute to exine structure and function (Scott
et al., 2004; Blackmore et al., 2007). The tapetum contribution to
exine synthesis and sporopollenin deposition starts while the
microspores are still attached in tetrads and continues through
the vacuolated stages until the first pollen mitosis is almost
completed (Blackmore et al., 2007). The spatio-temporal patterns of ACOS5 gene expression, revealed by in situ hybridization and promoter-reporter fusion expression (Figure 8), are
consistent with transient, tapetum-localized functions (occurring
maximally at around stage 7 of anther development) at the time of
tetrad formation and microspore release, when biosynthesis and
secretion of sporopollenin precursors is required. Furthermore,
the phenotype associated with loss of ACOS5 function in the
acos1 mutant first appears at stage 8 (Figures 6 and 7), consistent with defects in deposition of a critical secreted sporopollenin
component(s), leading to defective microspores. These microspores, when released from tetrads in stage 8 anthers, fail to
develop normal exine and are aborted in development by stage
9. Taken together, these data support the hypothesis that the
enzyme encoded by ACOS5 is required for production of sporopollenin constituents in the early steps of exine formation.
Despite the wealth of information on postmeiotic anther and
pollen development, only a small number of genes encoding
enzymes required for sporopollenin biosynthesis have been
identified by genetic approaches. The MS2 gene, defective in
the ms2 mutant, encodes a protein with similarity to a number of
fatty acyl reductases, including those with activity against fatty
acids and polyketides (Aarts et al., 1997), while the CYP703A2
gene (defective in dex2 mutants) encodes an in-chain fatty acid
hydroxylase with an in vitro preference for lauric acid (Morant
et al., 2007). Both CYP703A and MS2 are tightly coexpressed
with ACOS5 (see Supplemental Table 3 online; Morant et al.,
2007), and the similarities between the phenotypes of loss-offunction acos5, ms2, and dex2 mutants, coupled with the deduced biochemical functions of the corresponding proteins,
suggest that they could function in a common biochemical
pathway required for sporopollenin monomer biosynthesis. A
unique aspect of the acos5 phenotype, however, is its complete
male sterility relative to the low levels of fertility of ms2 and dex2
mutants, despite apparent similar defects in sporopollenin deposition and exine formation in all three mutants. One possible
explanation for this phenotypic difference is that the ACOS5
protein could act more centrally in a sporopollenin biosynthetic
pathway than MS2 and CYP703A2, such that the acos5 mutant is
completely devoid of sporopollenin, while ms2 and dex2 mutants
retain low, undetected levels of a modified polymer allowing
some pollen function to be retained. Thus, in contrast with the
interpretation of the results obtained by Morant et al. (2007), our
data suggest that sporopollenin deposition during exine formation may be absolutely required for pollen development and seed
production in Arabidopsis and that a complete block in the
sporopollenin biosynthetic pathway in a mutant such as acos5
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Figure 9. Competition Assays to Assess Substrate Preference of the ACOS5 Acyl-CoA Synthetase.
(A) Short- to medium-chain fatty acids.
(B) Medium- to long-chain fatty acids.
(C) Hydroxylated C8 fatty acids.
(D) Hydroxylated medium- to long-chain fatty acids.
(E) Hydroxycinnamic acids.
Activity of ACOS5 was determined under standard conditions with radiolabeled oleic acid (20 mM) in the absence or presence of the unlabeled
competing substrates (each added at 50 or 100 mM). Unlabeled oleic acid (C18:1) was used as positive control, while 5-dodecanolide served as a
ACOS5 in Pollen Wall Biosynthesis
leads to complete loss of pollen function and viability prior to
anther dehiscence.
An alternative hypothesis for the function of ACOS5 is participation in a vital tapetum-specific metabolic pathway not directly
involved in exine formation and sporopollenin biosynthesis, but
instead required for basic tapetal cell development and function.
In this case, loss of ACOS5 function would indirectly lead to
aberrant pollen wall formation and abortion of pollen grain
development due to a basic biochemical or developmental
defect in tapetal cells. However, as shown in Figure 7, we did
not observe any obvious aberrations in the appearance of the
tapetum in the acos5 mutant, suggesting that tapetal function
itself is not strongly affected in the mutants (Figure 7). In addition,
our in situ hybridization results showed that ACOS5 expression in
the tapetum is transient and restricted largely to stages 7 and 8 of
anther development, returning to background levels well before
initiation of normal tapetal degeneration at stages 10 and later.
This expression pattern, coupled with the apparently normal
tapetal cell morphology in the acos5 mutant through state 9, is
consistent with a role for ACOS5 in a pathway specific for the
biosynthesis of sporopollenin monomeric constituents required
for exine formation, rather than in the functioning of the tapetum
itself.
A number of genes encoding enzymes involved in fatty acid
metabolism have been shown to be expressed in tapetal cells
over the course of anther development, which like ACOS5 could
contribute to sporopollenin biosynthesis. For example, in addition to the MS2 and CYP703A2 genes discussed above, the
wheat (Triticum aestivum) TAA1 gene encodes a putative tapetumlocalized fatty acyl reductase that is capable of generating
medium- to long-chain fatty alcohols in tobacco and E. coli
(Wang et al., 2002), suggesting that it could work downstream of
a fatty acyl-CoA synthetase, such as ACOS5. Wang and Li (2008)
recently identified a cotton (Gossypium hirsutum) acyl-CoA synthetase, ACS1, that is required for male fertility and pollen
development in the early stages of microsporogenesis in cotton.
However, while expressed in cotton tapetal cells, ACS1 has a
broader expression domain than ACOS5 that includes microspores themselves. Furthermore, loss of ACS1 function affects
microspore development at a much earlier stage than the acos5
mutant, with defects in microspore development observed at the
pollen mother cell and tetrad stages, resulting in release of
aberrant unicellular microspores (Wang and Li, 2008). Finally, we
regenerated the protein alignment shown in Supplemental Figure
3A online to include Gh ACS1 and Gh ACS2 (see Supplemental
Figure 3B online). A phylogenetic reconstruction of these aligned
sequences (see Supplemental Figure 4 and Supplemental Data
Set 1 online) indicates that these two cotton acyl-CoA synthe-
519
tases are not closely related to the clade A ACOS gene family, but
instead are most closely related to Arabidopsis LACS4, LACS5,
and other LACS proteins, with which they share a characteristic
LACS linker region absent in 4CL and ACOS proteins (see
Supplemental Figure 4B online). This suggests that, in contrast
with ACOS5, these cotton enzymes are involved in long-chain
fatty acid metabolism required for the early stages of microspore
development and unlike ACSO5 may not be directly involved in
sporopollenin biosynthesis.
ACOS5 Is an Ancient Acyl-CoA Synthetase Conserved in
Land Plants with in Vitro Preference for Hydroxylated
Medium- to Long-Chain Fatty Acyl Substrates
Arabidopsis contains a large set of adenylate-forming acylactivating enzymes of diverse functions (Shockey et al., 2003),
including four 4CLs active toward hydroxycinnamic acids (Ehlting
et al., 1999; Hamberger and Hahlbrock, 2004), nine LACS active
toward long-chain fatty acids (Shockey et al., 2002), and
O-succinylbenzoyl-CoA ligase (Kim et al., 2008). Within the 4CL-like
clade of Arabidopsis ACOS enzymes (de Azevedo Souza et al.,
2008), the peroxisomally localized OPC-8:CoA ligase (OPCL1) is
involved in chain shortening reactions in jasmonic acid biosynthesis (Koo et al., 2006; Kienow et al., 2008). Previous work
showed that the ACOS5 gene encodes a likely nonperoxisomal
acyl-CoA synthetase, closely related to true 4CLs but of unknown
biochemical or developmental function (Shockey et al., 2003; de
Azevedo Souza et al., 2008). As illustrated in Figure 1, similar
enzymes are evolutionarily conserved in angiosperms as well as
in Physcomitrella, suggesting an ancient and conserved function
for this class of genes, which is preferentially expressed in tapetal
cells in both Arabidopsis (Figure 8) and tobacco (Varbanova et al.,
2003) and preferentially in male flowers in poplar (de Azevedo
Souza et al., 2008).
Initial analysis of the recombinant ACOS5 protein suggested a
lack of activity against hydroxycinnamic acids, substrates typically used by 4CLs (Costa et al., 2005; Kienow et al., 2008), while
work reported here (Table 2) demonstrated in vitro activity
against oleic acid (C18:1) with kinetic constants comparable to
those of other fatty acyl-CoA synthetases. Hydroxycinnamic
acids failed to compete with oleic acid as ACOS5 substrates
(Figure 9), verifying that ACOS5 is not a 4CL. These data suggest
that the in vivo substrate of ACOS5 is a fatty acid, consistent with
a role for ACOS5 in the biosynthesis of an aliphatic sporopollenin
monomeric constituent. Moreover, competition assays showed
an in vitro preference of ACOS5 for medium-chain fatty acids,
including hydroxy fatty acids that appear to be important sporopollenin constituents, since they provide the second functional
Figure 9. (continued).
negative control. Other competing substrates were as follows: hexanoic acid (C6), octanoic acid (C8), decanoic acid (C10), dodecanoic acid (C12),
tetradecanoic acid (C14), hexadecanoic acid (C16), octadecanoic acid (C18), oleic acid (C18:1), 2-hydroxy octanoic acid (2OH-C8), 3-hydroxy octanoic
acid (3OH-C8), 8-hydroxy octanoic acid (8OH-C8), 10-hydroxy decanoic acid (10OH-C10), 12-hydroxy dodecanoic acid (12OH-C12), 16-hydroxy
hexadecanoic acid (16OH-C16), 12-hydroxy octadecanoic acid (12OH-C18), 3-oxo-2(29-[Z]-pentenyl)cyclopentane-1-octanoic acid (OPC-8), cinnamic
acid (CinA), coumaric acid (CouA), caffeic acid (CafA), ferulic acid (FerA), sinapic acid (SinA), and 5-dodecanolide (serving as control). All values are the
mean 6 SD of at least three determinations (100% corresponding to an activity of 0.35 nkat/mg protein).
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The Plant Cell
group required for the formation of the extensive ether crosslinks in the sporopollenin polymer (Ahlers et al., 2000, 2003;
Morant et al., 2007). Based on competition assays, the best
ACOS5 substrates among those tested were 16-hydroxy hexadecanoic acid (16OH-C16), 8-hydroxy octanoic acid (8OH-C8),
and 12-hydroxy octadecanoic acid (12OH-C18). Interestingly,
OPC-8, an intermediate in jasmonic acid biosynthesis containing
an 8-carbon fatty acyl tail, was also an effective competitor. This
suggests that recombinant ACOS5 can bind to and use a variety
of substrates in vitro, provided that the substrate fulfills key
structural requirements, such as having the polar group (oxygen)
in the fatty acid side chain 7 to 15 carbons distal to the carboxy
group, which can dramatically increase substrate usage.
While the in planta substrate of ACOS5 remains to be determined, and the lack of defective mature pollen grains in the acos5
mutant precluded analysis of mutant pollen wall composition,
this repertoire of potential substrates is consistent with the
deduced aliphatic constituents that form the bulk of cross-linked
sporopollenin monomers from several species (Piffanelli et al.,
1998; Dominguez et al., 1999; Ahlers et al., 2000; Bubert et al.,
2002). It is also consistent with in vivo activity of ACOS5 as an
acyl-CoA synthetase that uses a number of fatty acids ranging
from C8 to C18 when expressed in E. coli (Kienow et al., 2008).
The combined in vitro and in vivo data suggest that ACOS5 plays
a key role in the biosynthesis of the aliphatic monomeric con-
stituents of sporopollenin by forming fatty acyl-CoA esters
of polymer precursors, potentially including the CoA ester of
7-hydroxy lauric acid, which is proposed to be generated by
CYP703A2 (Morant et al., 2007). An analogous role for the
Arabidopsis LACS2 enzyme in biosynthesis of cutin, an aliphatic
polyether polymer related to sporopollenin, has been proposed
(Schnurr et al., 2004).
Model for the Role of ACOS5 in Sporopollenin Biosynthesis
The combination of our functional data for ACOS5 and similar
data for CYP703A and MS2 (Aarts et al., 1997; Morant et al.,
2007), and other enzymes localized to the tapetum, such as
TAA1 (Wang et al., 2002), provides further insight into potential
pathway(s) for generation of sporopollenin monomeric units.
Based on these data, we propose a working model for the
biosynthesis of sporopollenin monomers (Figure 10). According
to this model, the fatty acyl-CoA ester product of the ACOS5
catalyzed reaction is a central intermediate used to generate
sporopollenin monomers in tapetal cells for export to the locule.
Consistent with a central role for ACOS5 in one or more biochemical pathways leading to sporopollenin monomer biosynthesis is the strong sterility phenotype of the acos5 mutant and
the highly correlated coexpression of ACOS5 with Arabidopsis
genes encoding enzymes that could act in the same pathway(s),
Figure 10. Model for the Role of ACOS5 in Sporopollenin Monomer Biosynthesis in Developing Anthers.
Enzymes encoded by genes with known functions in pollen development and sporopollenin biosynthesis are indicated in grey. The potential cellular and
subcellular localization of enzymes in the developing anther is shown. According to this model, the fatty acyl-CoA ester product of the ACOS5 reaction
(for example, 7-hydroxylauryl-CoA formed by the action of CYP703A2) could be a central intermediate used to generate sporopollenin monomers by
direct transport of the CoA ester to the locule, reduction of the CoA ester to an aldehyde or alcohol followed by transport to the locule, and/or by further
modification of the fatty acyl-CoA starter and transport to the locule. CoASH, CoA; ER, endoplasmic reticulum.
ACOS5 in Pollen Wall Biosynthesis
including CYP703A2 and MS2 (Morant et al., 2007; data not
shown).
As shown in Figure 10, one function of ACOS5 could be to
regenerate the CoA ester of the proposed hydroxylated fatty acid
generated by CYP703A2 (7-hydroxylauryl-CoA). Such CoA esters are likely required for fatty acid export across the plasma
membrane (Schnurr et al., 2004); thus, a loss in ACOS5 activity
could prevent secretion of such a hydroxy-fatty acyl sporopollenin monomer. While the formal possibility exists that ACOS5
could function as a plastid localized acyl-CoA synthetase required to generate a fatty acyl-CoA ester for export into the
cytoplasm, there is no in silico evidence for such localization, and
an ACOS5:YFP fusion protein localized to the cytoplasm (Figure
2). Alternatively, or in addition, ACOS5 activity could be required
to generate a fatty acyl-CoA substrate for reduction (Figure 10).
For example, MS2 or other tapetum-expressed reductases could
generate fatty aldehyde or alcohol monomeric constituents of
sporopollenin from the ACOS5-derived CoA ester, which could
then be exported into the locule for incorporation into the sporopollenin polymer. Finally, the ACOS5-derived fatty acyl CoA
ester could also be used as a starter molecule for incorporation
into potentially more complex sporopollenin monomeric constituents, analogous to the incorporation of 4CL-derived p-coumarylCoA into flavonoids. In this way, the function of ACOS5 in
tapetal cells could be analogous to that of 4CL, which generates
hydroxycinnamyl-CoA esters used in distinct branch pathways.
Both CYP703A2 and ACOS5 are conserved in land plant
lineages, including Physcomitrella, but are absent in Chlamydomonas (Morant et al., 2007; de Azevedo Souza et al., 2008; Table
1, Figure 1). Thus, acquisition of an ACOS5- and CYP703A2dependent sporopollenin biosynthetic pathway appears to be an
adaptation that was shared by the common terrestrial ancestor of
bryophytes and vascular plants. The ability to generate sporopollenin was likely a key land plant innovation essential for
protection of haploid spores from desiccation, UV irradiation,
and other stresses of the terrestrial environment, and its evolution
likely predated vascular system development and the ability to
produce lignin (Bowman et al., 2007). Thus, it is conceivable that
the repertoire of 4CL and 4CL-related enzymes now found in land
plants (Figure 1; de Azevedo Souza et al., 2008) arose from an
ACOS5-like ancestral enzyme. Further definition of the biochemical pathway involving ACOS5 should not only reveal the nature of
sporopollenin monomeric constituents and the sporopollenin
polymer but also shed light on the evolution of the diverse
polyether and polyester polymers now found in plants.
METHODS
Plant Material and Growth Conditions
Arabidopsis thaliana wild-type (Col-0) and mutant plants were grown in
soil (Sunshine mix 5; Sungrow Horticulture) in controlled environment
chambers at 208C under long-day conditions (18 h light).
Identification and Characterization of an ACOS5 Insertion Mutant
An Arabidopsis line with a transposon insertion in ACOS5 (N123936;
synonymous with SM_3.37225) was obtained from the NASC and ABRC.
Homozygous plants were identified by PCR using primers CLL4F and
521
EcoRI reverse (see Supplemental Table 4 online) for detection of the
endogenous gene, and CLL4F and dspn1 for detection of the transposon
insertion. Homozygous plants were identified by PCR using primers 5RP
and 5LP in combination with the T-DNA left-border primer LB1 (see
Supplemental Table 4 online).
Crosses of wild-type pollen to homozygous acos5 mutant plants were
performed to obtain F2 generation plants. The patterns of ACOS5 transposon insertion allele segregation in the F2 generations were tested by x2
statistical analysis of observed phenotypes and genotypes using GraphPad software (http://graphpad.com/quickcalcs/chisquared1.cfm), with
expected values based on Mendelian segregation, observed values on
the F2 population, and two degrees of freedom (genotypes) or one degree
of freedom (phenotypes). The two-tailed P value of >0.4 suggested that
the deviation of observed values from expected values was not significant.
Complementation of acos5 Mutants
A 4368-bp ACOS5 genomic fragment was amplified using the Plantinum
Taq DNA polymerase High Fidelity (Invitrogen) with gene-specific primers
(see Supplemental Table 4 online) and cloned into pCR8/GW/TOPO
(Invitrogen). After verification by sequencing, the fragment was subcloned
into the pGWB1 Gateway binary vector (Nakagawa et al., 2007) and
introduced into Agrobacterium tumefaciens. Then, acos5 heterozygous
plants were transformed using the floral dip method (Clough and Bent,
1998). T1 seeds were sown in half-strength Murashige and Skoog salts
(Sigma-Aldrich), supplemented with 1% sucrose and 0.6% agar medium
containing 25 mg/L hygromycin. Individual T1 lines were allowed to selfpollinate, and progeny genotypes were tested with respect to the ACOS5
locus, and the presence of respective transgenes was tested using PCR
with the primers given in Supplemental Table 4 online.
Nucleic Acid Methods
Genomic DNA extraction was performed using young leaf tissue ground
in a bead beater at 48C, with the use of the Nucleon PhytoPure Kit
(Amersham-Pharmacia), according to the manufacturer’s instructions.
Arabidopsis RNA was isolated from tissues frozen in liquid nitrogen and
ground to a fine powder using Trizol reagent (Gibco-BRL) following the
manufacturer’s instructions.
RT-PCR
RNA quality was assessed by visual inspection of rRNA on a 1.2%
formaldehyde-agarose gel and quantified spectrophotometrically, and
2.5 mg RNA/20 mL reaction was used to generate first-strand cDNA using
Superscript II Reverse Transcriptase (Invitrogen) following the manufacturer’s protocol. For RT-PCR analysis of ACOS5 expression in T-DNA
insertion lines, gene-specific and intron-spanning primers (see Supplemental Table 4 online) flanking the sites of the insertions were used in PCR
reactions to amplify corresponding cDNA sequences under the following
PCR conditions: 958C for 3 min, 30 cycles of (948C for 30 s, 568C for 30 s,
and 728C for 1 min), and 728C for 10 min, using Taq polymerase in a 50-mL
reaction. PCR products were separated on 1% ethidium bromide agarose gels and photographed under a UV transilluminator using AlphaImager 1220. Actin1 was used as the control.
Transient Expression and Subcellular Localization of ACOS5
in Planta
The cDNA of ACOS5 was PCR amplified using the primer pair attb1fw62940 and attb2rv-62940 (see Supplemental Table 4 online) and inserted
into the expression vector pENSGYFP, resulting in an N-terminal fusion
with YFP using the Gateway cloning system (Invitrogen) as previously
described (Schneider et al., 2005). The dsRED fluorescent protein with
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The Plant Cell
the C-terminal extension –SRL served as the peroxisomal marker protein
(Schneider et al., 2005). Both vectors allow the in planta expression of
proteins under the control of the constitutive cauliflower mosaic virus
double 35S promoter. For transient in planta expression, detached leaves
of 4-week-old Arabidopsis (Col-0) plants were bombarded with 1-mm
gold particles coated with vector DNA using the Biolistic PDS-1000/He
Particle Delivery System (Bio-Rad Laboratories) and subsequently placed
in a growth chamber for 36 h (Schneider et al., 2005). Fluorescence
microscopy was performed using a LSM 510 Meta confocal laser microscope (Carl Zeiss). An argon laser was used as the excitation source (514
nm), and light emission was detected in the range of 570 to 634 nm for
RFP constructs and 535 to 545 nm for YFP constructs. Images were
recorded and processed using LSM 510 3.2 software (Carl Zeiss).
Generation and Analysis of GUS Reporter Lines
GUS reporter lines of gene ACOS5 were created using Gateway technology (Invitrogen). Promoter sequences of 1620, 1416, and 1030 bp in
length were PCR amplified from Arabidopsis (Col-0) genomic DNA using
primer pairs Pfw1-62940 and Prv2-62940, Pfw3-62940 and Prv1-62940,
and Pfw2-62940 and Prv1-62940, respectively (see Supplemental Table 4
online) and cloned into vector pJawohl11 upstream of uidA (GUS). The
resulting plasmids were transferred into Arabidopsis (Col-0) plants by
Agrobacterium-mediated transformation (Clough and Bent, 1998). Transformants were selected on kanamycin-containing medium, and lines
homozygous for a single insertion were identified in the T2 generation.
These lines were grown in soil and liquid Murashige and Skoog medium
and inspected for GUS expression at different developmental stages and
after various treatments. For histochemical staining, seedlings or detached organs were placed in the GUS substrate solution (50 mM sodium
phosphate buffer, pH 7.0, 0.1% Triton X-100, 3 mM potassium ferricyanide, 3 mM potassium ferrocyanide, and 1 mM 5-bromo-4-chloro-3indolyl b-D-glucuronide), and after 5 min of vacuum infiltration, samples
were incubated overnight at 378C and subsequently destained by a series
of washes in 80% ethanol (Jefferson, 1987).
Expression and Enzymatic Analysis of Recombinant ACOS5
Heterologous expression in Escherichia coli strains DH5a or M15 and
purification of recombinant ACOS5 was performed as previously described (Stuible et al., 2000; Schneider et al., 2005). Enzyme purity was
inspected by SDS gel electrophoresis, and protein concentrations were
determined according to the Bradford method (Bradford, 1976) with BSA
as standard.
Enzymatic activity of the recombinant protein was determined by a
luciferase-coupled assay, which records the consumption of ATP, as
previously described (Schneider et al., 2005). For detailed kinetic analyses, a more sensitive and accurate radioactive assay using [1-14C]oleic
acid (specific activity 50 to 62 mCi mmol21; GE Healthcare, Amersham
Biosciences) was applied as previously described (Shockey et al., 2002).
Briefly, the assay mixture (volume of 100 mL) contained 100 mM Tris-HCl,
pH 7.6, 10 mM MgCl2, 5 mM ATP, 2.5 mM DTT, 1 mM CoASH, 20 mM oleic
acid (dissolved in 0.1% Triton X-100), and 2 mg of affinity purified protein .
The assay was initiated by addition of CoASH (CoA) and incubated at
room temperature for 10 min. The reaction was stopped by addition of
100 mL of 10% (v/v) acetic acid in isopropanol and extracted twice with
900 mL of hexane (saturated with 50% [v/v] isopropanol). Formation of the
oleoyl-CoA ester was quantified in the aqueous phase by liquid scintillation counting, and the identity of the product was confirmed by mass
spectrometry (see Supplemental Table 2 online). Km and Vmax values were
obtained by linear regression of v/s against s (Hanes plots) from at least
three independent experiments, in which the corresponding substrate
concentrations were varied appropriately. For substrate competition
assays, different fatty acids and cinnamic acid derivatives were added
to the standard reaction mixture at 50 or 100 mM, and formation of oleoylCoA was quantified as above. Hydroxy fatty acids and CoA were obtained
from Sigma-Aldrich and Fluka.
Phylogenetic and Bioinformatic Analyses
The set of Arabidopsis 4CL genes (Ehlting et al., 1999) was used in
reciprocal BLAST searches to identify ACOS5 homologs as previously
described by de Azevedo Souza et al. (2008). All sequences obtained are
given in Table 1 and Supplemental Table 1 online. Protein sequences
were aligned using the Genomatix DIALIGN program (http://www.
genomatix.de/cgi-bin/dialign/dialign.pl) (Morgenstern, 1999), and the
multiple protein sequence alignments were manually optimized. Aligned
sequences are shown in Supplemental Figure 3 online. Amino acid
sequences used to generate these alignments are shown in Supplemental Data Set 1 online. To reconstruct phylogenetic trees, maximum
likelihood analyses with 1000 bootstrap replicates were performed using
PhyML v2.4.4 (Guindon and Gascuel, 2003) with the JTT model of amino
acid substitution.
Phenotypic Analyses
Scanning electron microscopy was performed using a Hitachi S4700
microscope. Wild-type and homozygous mutant inflorescences were
fixed overnight in 2% glutaraldehyde, washed and postfixed in 1%
osmium tetraoxide in 0.05 M PIPES buffer, pH 7.0, and dehydrated using
a series of graded ethanol solutions (30 to 100%). Dried samples were
gold sputter coated (Nanotech SEMPrep II sputter coater). To obtain
cross sections of developing anthers, wild-type and homozygous mutant
inflorescences were fixed in FAA (4% paraformaldehyde, 15% acetic
acid, and 50% ethanol) overnight and directly dehydrated without
postfixation. Samples were then transferred to a propylene oxide solution
and slowly infiltrated with Spurr’s epoxy resin (Canemco). For bright-field
microscopy, 1-mm sections were cut with glass knives (Leica) on a
microtome, mounted on glass slides, heat fixed to the slides, and stained
with toluidine blue. Sections were photographed using a light microscope. All microscopy procedures described were performed in the UBC
BioImaging Facility (http://www.emlab.ubc.ca).
For transmission electron microscopy, Arabidopsis wild-type and
acos5 mutant inflorescences were fixed with paraformaldehyde and
postfixed in osmium tetraoxide. Fixed samples were dehydrated in an
ethanol gradient up to 100% and then transferred to a propylene oxide
solution and slowly embedded in Spurr’s resin and allowed to polymerize
for at least 48 h. Thin sections (70 nm) were taken using a diamond knife
microtome (Reichert Ultracut E). Sections were placed on 100-mesh
copper grids and stained for 30 min with uranyl acetate and for 15 min
with lead citrate (Sato’s Lead). Sections were visualized using a Hitachi
H7600 transmission electron microscope.
In Situ Hybridization
Arabidopsis Col-0 inflorescences were embedded in Paraplast (SigmaAldrich), sectioned at 8 mm thickness, and mounted onto precharged
slides. For sense and antisense ACOS5 probe synthesis, 526-bp DNA
template corresponding to the ACOS5 coding region was PCR amplified
from flower cDNA using gene-specific forward and reverse primers (see
Supplemental Table 4 online). A T7 polymerase binding site was incorporated into the forward primer for sense probe amplification and in the
reverse primer for antisense probe amplification. Digoxigenin-labeled
probes were transcribed off the template using T7 polymerase (Roche).
Probes were shortened to 200-bp fragments by limited carbonate hydrolysis, quantified, and hybridized to slides. Tissue fixation, embedding,
hybridization, and signal detection are described by Hooker et al. (2002).
ACOS5 in Pollen Wall Biosynthesis
Accession Numbers
Sequence data from this article can be found in the Arabidopsis Genome
Initiative or GenBank/EMBL databases under the following accession
numbers: Arabidopsis ACOS5, At1g62940; Arabidopsis Actin1, At2g37620.
Sequence data for all other genes described in this article can be found in
the relevant databases using the accession numbers or gene models
given in Supplemental Table 1 online.
Supplemental Data
The following materials are available in the online version of this article.
Supplemental Figure 1. Scanning Electron Micrographs of WildType (Col-0) and acos5 Mutant Anthers.
Supplemental Figure 2. Mass Spectrometric Analysis of CoA-Esters
Formed in Vitro by ACOS5.
Supplemental Figure 3. Alignment of Amino Acid Sequences Used
to Generate the Phylogenetic Tree Shown in Figure 1 and Supplemental Figure 4A Online.
Supplemental Figure 4. Relationship of ACOS5 and Clade A AcylCoA Synthetase Proteins to Cotton ACS1 and ACS2 Proteins.
Supplemental Table 1. Genes Used for Phylogenetic Reconstruction
Shown in Figure 1.
Supplemental Table 2. Molecular Masses of CoA-Ester Products
Formed in Vitro from Different Fatty Acids by ACOS5.
Supplemental Table 3. Coexpression of CYP703A2 and MS2 with
ACOS5.
Supplemental Table 4. Primer Sequences Used for PCR and RNA
Probe Generation.
Supplemental Data Set 1. Sequences Used to Generate the Phylogeny Presented in Figure 1 and Supplemental Figure 4A Online.
ACKNOWLEDGMENTS
We thank Bjoern Hamberger and Michael Friedmann for advice and
support. We also acknowledge the expert mass spectrometry analyses
performed by Thomas Colby. This work was supported by a Discovery
Grant from the Natural Sciences and Engineering Research Council of
Canada (NSERC) to C.J.D., by an NSERC Discovery Grant to G.W.H.,
and by the Deutsche Forschungsgemeinschaft (Ko1192/6-3) and the
Max Planck Society to E.K.
Received August 8, 2008; revised December 4, 2008; accepted January
27, 2009; published February 13, 2009.
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A Novel Fatty Acyl-CoA Synthetase Is Required for Pollen Development and Sporopollenin
Biosynthesis in Arabidopsis
Clarice de Azevedo Souza, Sung Soo Kim, Stefanie Koch, Lucie Kienow, Katja Schneider, Sarah M.
McKim, George W. Haughn, Erich Kombrink and Carl J. Douglas
PLANT CELL 2009;21;507-525; originally published online Feb 13, 2009;
DOI: 10.1105/tpc.108.062513
This information is current as of April 1, 2009
Supplemental Data
http://www.plantcell.org/cgi/content/full/tpc.108.062513/DC1
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